(1) Field of the Invention
This invention relates to a capacity control valve for a variable displacement compressor, and more particularly to a capacity control valve for use in a variable displacement compressor for compressing a refrigerant gas in a refrigeration cycle of an automotive air conditioner.
(2) Description of the Related Art
A compressor used for compressing refrigerant in a refrigeration cycle of an automotive air conditioner is driven by an engine, and hence is not capable of controlling the rotational speed thereof. For this reason, a variable displacement compressor capable of changing the compression capacity for compressing refrigerant is employed so as to obtain adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In such a variable displacement compressor, compression pistons are connected to a wobble plate fitted on a shaft driven for rotation by the engine, and the angle of the wobble plate is changed to change the stroke of the pistons for changing the discharge amount of the refrigerant, i.e. the capacity of the compressor.
The angle of the wobble plate is continuously changed by introducing part of the compressed refrigerant into a gastight pressure-regulating chamber and changing the pressure of the introduced refrigerant, thereby changing a balance between pressures applied to the both ends of each piston.
To control the amount of refrigerant introduced into the pressure-regulating chamber of the variable displacement compressor, in a compression capacity control device described e.g. in Japanese Unexamined Patent Publication No. 2001-132650, there have been proposed a construction in which a capacity control valve is disposed between a discharge chamber and a pressure-regulating chamber of the variable displacement compressor, and an orifice is provided between the pressure-regulating chamber and a suction chamber, and a construction in which an orifice is provided between a discharge chamber and a pressure-regulating chamber, and a capacity control valve is disposed between the pressure-regulating chamber and a suction chamber.
Each of the capacity control valves opens and closes the communication between the chambers such that a differential pressure across the capacity control valve is maintained at a predetermined value, and the capacity control valve is implemented by a solenoid control valve capable of externally setting the predetermined value of the differential pressure by a current value. Thus, when the engine rotational speed increases, the capacity control valve between the discharge chamber and the pressure-regulating chamber is opened, or the capacity control valve between the pressure-regulating chamber and the suction chamber is closed, whereby the pressure introduced into the pressure-regulating chamber is increased to reduce the volume of refrigerant that can be compressed, while when the engine rotational speed decreases, the capacity control valve is reversely controlled such that the pressure introduced into the pressure-regulating chamber is decreased to increase the volume of refrigerant that can be compressed, whereby the pressure of refrigerant discharged from the variable displacement compressor is maintained at a constant level irrespective of the engine rotational speed.
In such a capacity control valve for a variable displacement compressor, to minimize the operating capacity of the compressor, it is necessary to maximize the amount of refrigerant introduced from the discharge chamber into the pressure-regulating chamber or minimize the amount of refrigerant introduced from the pressure-regulating chamber into the suction chamber, and inversely, to maximize the operating capacity of the compressor, it is necessary to minimize the amount of refrigerant introduced from the discharge chamber into the pressure-regulating chamber or maximize the amount of refrigerant introduced from the pressure-regulating chamber into the suction chamber. If an orifice is provided between the discharge chamber and the pressure-regulating chamber or between the pressure-regulating chamber and the suction chamber of the compressor, the flow rate of refrigerant passing through the orifice is restricted. Therefore, when the operation of the compressor is changed from the maximum capacity operation to the minimum capacity operation or vice versa, the orifice limits the flow rate of refrigerant flowing from the discharge chamber to the pressure-regulating chamber or from the pressure-regulating chamber to the suction chamber, which causes much time to taken in transition to the minimum capacity operation or to the maximum capacity operation.
To eliminate this inconvenience, there is proposed a capacity control valve for a variable displacement compressor in Japanese Patent Application No. 2001-224209 which is arranged between a discharge chamber and a pressure-regulating chamber and between the pressure-regulating chamber and a suction chamber, for opening and closing communication between the discharge chamber and the pressure-regulating chamber and communication between the pressure-regulating chamber and the suction chamber, in an interlocked manner. This capacity control valve for a variable displacement compressor has a three-way valve construction in which two valves are arranged respectively between the discharge chamber and the pressure-regulating chamber and between the pressure-regulating chamber and the suction chamber, and when one of the valves is closed, the other is opened in a manner interlocked therewith, whereas when the one is opened, the other is closed in a manner interlocked therewith. The three-way valve is configured such that the high pressure-side valve arranged between the discharge chamber and the pressure-regulating chamber and the low pressure-side valve arranged between the pressure-regulating chamber and the suction chamber have the same effective pressure-receiving area so as to enable them to be moved only by the differential pressure between the discharge pressure and the suction pressure without being influenced by the pressure from the pressure-regulating chamber, and respective cross-sectional areas of refrigerant passages of the valves are made sufficiently larger than those of orifices. This makes it possible to cause a sufficiently large amount of refrigerant to flow during transition to the minimum capacity operation and the maximum capacity operation, which makes it possible to reduce the time taken for the transition.
Especially, when the compressor is operating in a state close to the minimum capacity operation, the refrigerant discharged from the discharge chamber is always introduced into the pressure-regulating chamber, so that the introduced refrigerant sometimes stays within the pressure-regulating chamber. In this state, to make a transition to the maximum capacity operation, it is desired to reduce the pressure within the pressure-regulating chamber as soon as possible. However, when the pressure-regulating chamber is communicated with the suction chamber to undergo a pressure drop in the pressure-regulating chamber, the refrigerant staying inside the pressure-regulating chamber is evaporated, and as long as the evaporation continues, the minimum capacity operation is maintained. Thus, it sometimes takes much time before the pressure in the pressure-regulating chamber actually drops. Even in such a case, since the three-way valve having large cross-sectional areas of the refrigerant passages fully opens the communication between the pressure-regulating chamber and the suction chamber, so that the refrigerant in the pressure-regulating chamber can be caused to promptly flow into the suction chamber, thereby reducing the time for transition from the minimum capacity operation to the maximum capacity operation.
However, although the high pressure-side valve and the low pressure-side valve of the conventional capacity control valve for a variable displacement compressor have the same effective pressure-receiving area, during most of actual operation, the valves are controlled such that the high pressure-side valve is fully closed and the low pressure-side valve is almost fully opened. Now, let it be assumed that the cross-sectional area of a valve hole of the high pressure-side valve is represented by A, the average cross-sectional area of a refrigerant passage of this valve when it is open by a, the cross-sectional area of a valve hole of the low pressure-side valve by B, and the average cross-sectional area of a refrigerant passage of this valve when it is open by b, the effective pressure-receiving area of the high pressure-side valve is represented by A−a, and the effective pressure-receiving area of the low pressure-side valve by B−b. During most of control time of actual operation, the effective pressure-receiving area of the high pressure-side valve is approximately equal to A, and that of the low pressure-side valve is equal to B−b, so that the high pressure-side valve and the low pressure-side valve are made to be different in effective pressure-receiving area, which causes the capacity control valve to be affected by the pressure from the pressure-regulating chamber.